Numerous conventional techniques exist for fabricating transistors, such as FETs, and integrated circuits containing FETs. See commonly assigned U.S. Pat. No. 6,465,836 and prior art referred to therein (the “'836 patent”). Similarly, there exist numerous conventional IC techniques for fabricating non-volatile, erasable memories, such as EEPROMs (“flash memories”), the data storage and transduction capabilities of which are not based on transient operation, as is the case with bipolar junction transistors. Moreover, the fabrication of split gate FET EEPROMs fabricated by IC techniques is known.
A split gate FET EEPROM is similar to a conventional FET EEPROM. Similar to the latter, it includes a source/drain channel region, which has formed thereover a control or select gate overlying the channel. The control gate includes a control gate electrode separated from the channel by a control gate dielectric or oxide. A split gate EEPROM includes the control gate and also includes an electrically “floating” gate that overlies only a portion of the channel. An electrode of the floating gate is separated from the channel by a tunneling dielectric or oxide layer. The tunneling dielectric layer permits the passage therethrough of carriers (electrons or holes) by Fowler-Nordheim (“FN”) tunneling and hot carrier injection. The floating gate electrode is beneath the control gate electrode and is separated therefrom by one or more insulative or dielectric layers.
Selected voltages are applied to the control gate electrode and to the source/drain to induce charge, reduce charge or sense charge in the floating electrode in order to write, erase or read the charge therein.
Prior techniques for fabricating FET EEPROMs are complicated and costly and often produce memories that do not operate appropriately, as noted in the '836 patent.
FinFETs are also known in the art. A FinFET includes an extended semiconductor fin that is elevated above a substrate in a direction normal to the plane of the substrate. Electrically continuous gates are fabricated on both sides of the fin and overlie both sides of a channel region defined between a source/drain that is formed in the fin, typically by ion implantation followed by rapid thermal annealing (“RTA”). It may be said that a FinFET includes a “double gate,” one on either side of the channel in the fin. See the '836 patent.
FET technology is presently dominant in the fabrication of transistors, memories and other devices. Performance enhancement in more recent generations of devices is generally achieved by reducing device size, often termed “scaling,” which results in faster device speed. However, as FETs are scaled to possess channel lengths less than 100 nm, their conventional stacked or horizontal orientation may lead to several problems, including unwanted coupling or interactions between the source and drain, that degrade the ability of the gate to turn the device “on” or “off.” This degradation is often referred to as the “short channel effect,” or SCE.
FETs fabricated by semiconductor-on-insulator, or “SOI,” techniques are typically formed on an insulative layer covering a semiconductor layer, unlike “bulk” FETs, which are formed directly on substrates. SOI techniques have been found to reduce unwanted coupling between the source and the drain, because all of the semiconductor in the channel region can be inverted or depleted by the gate. However, as further scaling has occurred and the distance between the source and the drain has been reduced, interactions among the source, drain and the channel have increased, exacerbating SCE. The double gate of a FinFET allows control of the channel from both of its (and the fin's) sides and has been found to reduce SCE. Moreover, when the device is turned “on” using both gates, two conduction or inversion layers are formed in the channel, allowing for increased current flow therein.
In a recently developed FinFET structure, the gate straddles or wraps around the fin so that it nearly completely surrounds the channel. This has been found to further enhance gate control. See U.S. Pat. No. 6,413,802 (the “'802” patent) and prior art referred to therein
A combined EEPROM-FinFET structure is shown by US Published Application 2003/0042531 (the “'531 publication”). This combination is intended to take advantage of the benefits of both types of devices as scaling continues. However, a split gate EEPROM structure is not implemented in the device of the '531 publication.
The present invention contemplates the convenient and expedient combination of FinFET technology and split gate EEPROM technology in an SOI device.